Haha - YES it is possible to end up with full sized trees if you don't prune. If you would like to use branches from full sized trees, you can, but must keep pruning back hard every spring. For smaller plants, you might try a dwarf fruit tree and espalier it along the edges, a grapevine also might work. Non-fruiting plants that bend like this include dogwood, forsythia, smokebush or even something like tree peony. Any plant that keeps it's woody stems through the winter, but puts out new flexible stems each spring will work!

Another note --these little fences will last several years if pruned, but will probably not last forever and ever :)